1. Field of the Invention
The present invention relates to signal amplifiers and, more particularly, to a low noise transimpedance preamplifier circuit employing capacitive feedback to compensate for an undesired bandwidth roll-off.
2. Description of the Related Art
Low noise preamplifier circuits are commonly used in instrumentation and communication applications to provide signal gain with minimal added noise, thus maintaining a high signal-to-noise ratio. The smaller the desired signal which is applied to the amplifier, the more critical it is that the signal-to-noise ratio is properly maintained.
A typical preamplifier circuit for use in an optical receiver circuit front end is illustrated in FIG. 1. Referring to FIG. 1, a photodetector 10 is coupled to an input of an amplifier circuit 12. The photodetector 10 includes a photodiode 10a having an associated parasitic capacitance 10b. The value of the parasitic capacitance 10b is proportional to the size of the photodetector 10. When connected to the preamplifier circuit, the capacitor 10b creates a loading effect which limits the frequency response of the preamplifier circuit.
The circuit of FIG. 1 includes a resistor 14, which is connected across the amplifier circuit 12 from the input to the output. In transimpedance amplifiers, the resistor 14 is referred to as a transimpedance resistance and, in part, determines the gain and noise properties of the preamplifier circuit 12. By reducing the value of the resistor 14, the loading effects of capacitor 10b can be reduced. However, reducing the value of the transimpedance resistor 14 also results in increased noise which reduces the signal-to-noise ratio, and therefore, the sensitivity, of the preamplifier circuit 12.
To avoid the undesired effects associated with reducing the transimpedance resistor 14, other circuits have been used which employ post-amplification equalization. Such circuit topologies are generally referred to as high-impedance amplifiers, or integrating front end amplifiers. These circuits employ a very high impedance input circuit which, when combined with an input capacitor such as 10b, results in an integration function. The combination of the high input impedance and parasitic capacitor 10b severely limits the frequency response of these amplifiers. Thus, an equalization circuit must be employed after the integrating amplifier in order to restore the required frequency response. In order to be effective, the time constants of the equalization circuit must be precisely selected and maintained. Therefore, this technique is complicated and expensive to implement, therefore, rendering it prohibitive for many applications.
Accordingly, there remains a need for a low noise preamplifier circuit which employs a simple circuit topology to compensate the frequency response of the amplifier in the presence of an applied input capacitance.